Container and method for maintaining stability of gas mixtures

ABSTRACT

A container for holding a single constituent gas or a mixture of constituent gases is disclosed. The container has a wall structure defining an internal space. An opening in the wall structure permits access to the internal space. A closure is positioned over the opening. A seal attaches the closure to the wall structure maintains the single constituent gas or the constituents of the gas mixture at a predetermined concentration over a predetermined time period. A valve is mounted on the closure for permitting controlled release of the mixture. A method of maintaining gas concentration stability by forming a seal between the closure and the wall structure is also disclosed.

FIELD OF THE INVENTION

This invention concerns a method and a container adapted to maintain asingle gas or constituent gases of a gas mixture within a predeterminedconcentration range over a predetermined time period.

BACKGROUND OF THE INVENTION

Gas mixtures having precise concentrations of constituent gases are usedextensively in many industrial and laboratory processes. For example,mass spectrometers for identifying chemical compounds use a gas mixturecomprising bromopentafluorobenzene, trifluoromethyl benzene and VOC-freenitrogen for calibration purposes. These constituent gases are presentin the mixture at predetermined concentrations appropriate for thecalibration purposes. Proper operation of the mass spectrometer dependsupon its accurate calibration, which, in turn, depends upon the accuracyof the relative concentration of the constituent gases in thecalibration gas mixture.

When gas mixtures are held under pressure in containers, the relativeconcentration of the constituent gases may change over time, renderingthe mixture useless for its intended purpose. The mixture is said tosuffer from “instability”, which is defined as the tendency of theconstituent gases to change in concentration (either rise or fall) overtime. Long term instability of a gas mixture occurs when theconcentration of one or more constituent gases in the mixture changesafter several (4-6) months, while short term instability is said tooccur when the concentration of the constituent gases changes over aperiod of less than three months.

Instability is an especially acute problem for gas mixtures stored atrelatively low pressures and having relatively low concentrations ofreactive components. For example, the aforementioned gas mixture formass spectrometer calibration contains 50 ppmv ofbromopentafluorobenzene and 100 ppmv of trifluoromethyl benzene in 5liters of VOC-free nitrogen at 700 kPa at 21° C. In view of the smallamount of the active constituent gases in the mixture, anycharacteristic of the container that affects the concentration of theseconstituents will have a significant effect on both the long and shortterm stability of the mixture.

The container characteristics which have the greatest effect onstability include adsorption, absorption and leakage. Practicalcontainers typically comprise a pressure vessel, for example, athin-walled aluminum cylinder, with an opening that receives a caphaving a valve. For low pressure applications, the cap is crimped orswaged onto an outwardly curled lip that surrounds the opening. Anelastomeric gasket, such as an O-ring or washer, is positioned betweenthe cap and the lip to provide a fluid tight seal. The valve may bespring biased, and provides for controlled filling and discharge of thegas mixture.

Gas leakage occurs between the valve cap and the container lip becausethe swaging or crimping process does not form a continuous, fluid tightconnection between the cap and the lip. The use of the gasket betweenthe cap and the lip is intended to provide a fluid tight seal, but leakscan still occur between the gasket and the lip or the valve cap becauseit is not always possible to control the surface finish of the vessel orthe cap, and the gasket cannot conform and block the small surfaceirregularities that provide unacceptable leakage paths from thecontainer.

A leaky seal will not only allow gas to escape (thereby changing therelative concentration of the constituents), it may also allowcontaminant gases, such as oxygen, to diffuse into the vessel againstthe gas pressure. The oxygen combines with the reactive components,effectively removing them from the mixture and changing the constituentconcentration.

Even when the leakage rate is acceptable (i.e., the gas mixture remainsstable within acceptable limits), the use of elastomeric gaskets cancause instability by absorbing or adsorbing the more reactiveconstituent gases, effectively removing them from the mixture andthereby changing the relative concentration of the components. It isclear that the effects of leakage and absorption or adsorption will havea proportionally greater effect for smaller concentrations of theconstituent gases.

In addition to containers of gas mixtures, containers having a singlegas constituent, for example, pure nitrogen for purging or calibrationpurposes, may also suffer from instability. In the case of a single gasconstituent, the diffusion of contaminants into the container, such asoxygen or moisture, must be mitigated so that the concentration of theimpurities does not increase.

There is clearly a need for a container and a method of maintaining thestability of gas mixtures as well as single constituent gases, andespecially for gas mixtures stored at relatively low pressures (up to 18bar) and at relatively small concentrations (in the parts per million byvolume range).

SUMMARY OF THE INVENTION

The invention concerns a container for holding a mixture of a pluralityof constituent gases. The gases have respective predeterminedconcentrations within the mixture. The container comprises a vesselhaving a wall structure defining an internal space for containing thegas mixture. The wall structure has an opening therein for access to theinternal space. A closure is positioned in overlying relation with theopening. A seal is positioned between the closure and the wall structurefor attaching the closure to the wall structure so as to maintain afirst one of the gases at a concentration within a range of about +1% toabout −5% of the predetermined concentration of the first gas over abouta one month period. A valve is mounted on the closure. The valve isopenable and closable for permitting controlled release of the gasmixture from the vessel.

The first gas may be maintained at a concentration within a range ofabout +1% to about −7% of the predetermined concentration of the firstgas over about a one month period. The first gas may be maintained at aconcentration within a range of about +1% to about −5% of thepredetermined concentration of the first gas over about a one monthperiod. A second one of the gases may be maintained at a concentrationwithin a range of about +3% to about −15% of the predeterminedconcentration of the second gas over about a one month period. Thesecond gas may be maintained at a concentration within a range of about+3% to about −30% of the predetermined concentration of the second gasover about a one month period. A third one of the gases may bemaintained at a concentration within a range of about +3% to about −15%of the predetermined concentration of the third gas over about a onemonth period. The third gas may be maintained at a concentration withina range of about +3% to about −30% of the predetermined concentration ofthe third gas over about a one month period.

Preferably, the seal comprises a weld integrally joining the closure tothe wall structure. The weld is preferably formed by means of anelectron beam.

The invention further encompasses a container for holding a mixture of aplurality of constituent gases. The gases have respective predeterminedconcentrations within the mixture. The container is adapted to maintainthe concentrations of the gases within a predetermined concentrationrange over a predetermined time period. The container comprises a vesselhaving a wall structure defining an internal space for containing thegas. The wall structure comprises a metal having the property of beinginert with respect to the constituent gases and has an opening thereinfor access to the internal space. A closure is positioned in overlyingrelation with the opening. A seal for attaching the closure to the wallstructure is positioned between the closure and the wall structure. Theclosure and the seal have the same properties of being inert to theconstituent gases as the wall structure. A valve is mounted on theclosure. The valve is openable and closable for permitting controlledrelease of the gas mixture from the vessel.

The invention also includes container for holding a single constituentgas. The single constituent gas has a predetermined concentration ofimpurities upon filling. The container comprises a vessel having a wallstructure defining an internal space for containing the gas. The wallstructure has an opening therein for access to said internal space. Aclosure is positioned in overlying relation with the opening. A seal forattaching the closure to the wall structure so as to maintain theimpurities at a concentration within a range of about +1% to about −1%of the predetermined concentration of the impurities upon filling overabout a one month period. A valve is mounted on the closure. The valveis openable and closable for permitting controlled release of the gasmixture from the vessel.

The invention also includes a method of maintaining gas concentrationstability among constituent gases of a gas mixture within apredetermined concentration variation range and over a predeterminedtime period. The method comprises the steps of:

(a) providing a vessel for holding the gas mixture under pressure, thevessel having an opening therein;

(b) providing a closure adapted to engage the vessel in overlyingrelation with the opening, the closure having a valve mounted thereonfor permitting controlled filling and release of the gas mixture to andfrom the vessel;

(c) forming a seal between the closure and the vessel, the seal, theclosure and the vessel having the property of being inert with respectto the constituent gases, the seal providing a fluid tight joint betweenthe closure and the vessel that maintains a first one of the gases at aconcentration of about +1% to about −5% of a predetermined concentrationof the first gas over about a one month period.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a container for stablestorage of gas mixtures and single constituent gases according to theinvention; and

FIG. 2 shows a portion of FIG. 1 designated within a broken circle andon an enlarged scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a container 10 for holding single constituent gases or gasmixtures comprising a plurality of constituent gases havingpredetermined concentrations within the mixture. The container isadapted to maintain the concentrations of the gases within apredetermined concentration range over a predetermined time frame andthereby provide for the stable storage of the gas mixtures, both longand short term.

Container 10 comprises a pressure vessel 12, preferably formed ofaluminum and having a thin sidewall structure 14 that surrounds aninternal space 16. Aluminum is a preferred material because it isself-passivating and inert, and will not react with the constituentgases and change their concentration. Vessel 12 is preferablycylindrical in cross section and has a bottom 18 at one end and anopening 20 at the opposite end for access to the internal space 16.

A rim 22, best shown in FIG. 2, surrounds opening 20, the rim projectingoutwardly away from the central space. The rim provides a mountingfeature for a closure 24, and preferably comprises a curved lip 26 (seeFIG. 1). Lip 26 is formed by outwardly turning the sidewall 14 aroundthe perimeter of the opening 20. Turning the sidewall to form the lipresults in a substantially spiral cross section 28 and provides a rim 22having significant bending stiffness and strength, thereby reinforcingan inherently weak part of the vessel.

Closure 24 is preferably a circular aluminum cap 30 positioned inoverlying relation with the opening 20. Aluminum is preferred for theclosure because it is advantageous that the closure also have theproperty of being inert with respect to the constituent gases. Cap 30has a circumferential flange 32 adapted to engage the rim 22.Preferably, flange 32 has a curvature 34 that is substantially matchedto the curvature of lip 26. The curvature of the flange allows theclosure to center itself within the opening 20.

Closure 24 also includes a valve 36 mounted on the cap 30. Valve 36 ispreferably a spring biased valve and provides for the controlled fillingand release of gas from the internal space 16.

Closure 24 is welded to the lip 26 by a weld 38 extending between theflange 32 and the lip. The weld is preferably formed by an electron beam40 applied to the interface between the flange and the lip whilerotating the vessel around its longitudinal axis within an evacuatedchamber (not shown). Electron beam welding is preferred because thetechnique allows for significant heat energy to be preciselyconcentrated over a very small area with great accuracy, therebyallowing the weld 38 to be formed joining the closure to the vesselacross the relatively small target that is the flange 32. Excess heat isavoided, thereby protecting the polymer components of valve 36 fromdamage. If more thermal protection is required, a heat sink (not shown)may be attached to the valve during welding.

The weld 38 thus formed provides a substantially continuous fused metalseal between the closure and the vessel which does not permit anysignificant leakage or diffusion, thereby promoting stability of the gasmixture. The weld 38, being the same material as the closure and thevessel also has the property of being inert with respect to theconstituent gases. Use of the continuous weld as a seal obviates theneed for a gasket, thereby eliminating a source of absorption oradsorption that can adversely affect constituent gas concentration.

Gas containers according to the invention have been tested for stabilityof the mixtures. Specifically, containers having a mixture of 50 ppmvbromopentafluorobenzene, 100 ppmv trifluoromethyl benzene in 5 liters ofVOC-free nitrogen at 700 kPa at 21° C. were analyzed for stability overtime. It was found that the concentration of trifluoromethyl benzenevaried between about +1% and about −5% over a three month period, andbetween about +1% and about −7% over a six month period. In theanalysis, the concentration of bromopentafluorobenzene was found to varybetween about +3% to about −15% over a three month period and betweenabout +3% and about −30% over a six month period. These results showimproved stability over prior art containers that use elastomeric seals.In view of such test results, it is expected that the stability ofgases, with as many as three or more constituents, should be readilyachievable within the aforementioned concentration levels and withinabout a one month time period.

When containers according to the invention are used to hold singleconstituent gases, for example, pure nitrogen, it is recognized thatthere will be some impurities present in the gas upon filling of thecontainer. For stability of such single constituent gases, it ispreferable that the concentration of total impurities reported in theconstituent gas on the day of fill does not change by more than +/−1%over a period of about one month. For example, for a container of purenitrogen, the oxygen content must not change, that is, increase ordecrease, by 2 ppm or greater for the gas constituent to be consideredstable. Similarly, instability occurs when concentrations of thefollowing impurities change by the following amounts or more: totalhydrocarbons +/−0.05 ppm, carbon monoxide +/−0.1 ppm; carbon dioxide+/−0.3 ppm; and water vapor +/−2 ppm.

1. A container for holding a mixture of a plurality of constituentgases, said gases having respective predetermined concentrations withinsaid mixture, said container comprising: a vessel having a wallstructure defining an internal space for containing said gas mixture,said wall structure having an opening therein for access to saidinternal space; a closure positioned in overlying relation with saidopening, a seal for attaching said closure to said wall structure so asto maintain a first one of said gases at a concentration within a rangeof about +1% to about −7% of said predetermined concentration of saidfirst gas over about a one month period; and a valve mounted on saidclosure, said valve being openable and closable for permittingcontrolled release of said gas mixture from said vessel.
 2. A containeraccording to claim 1, wherein said first gas is maintained at aconcentration within a range of about +1% to about −5% of saidpredetermined concentration of said first gas over about a one monthperiod.
 3. A container according to claim 1, wherein a second one ofsaid gases is maintained at a concentration within a range of about +3%to about −15% of said predetermined concentration of said second gasover about a one month period.
 4. A container according to claim 1,wherein said second gas is maintained at a concentration within a rangeof about +3% to about −30% of said predetermined concentration of saidsecond gas over about a one month period.
 5. A container according toclaim 1, wherein a third one of said gases is maintained at aconcentration within a range of about +3% to about −15% of saidpredetermined concentration of said third gas over about a one monthperiod.
 6. A container according to claim 1, wherein said third gas ismaintained at a concentration within a range of about +3% to about −30%of said predetermined concentration of said third gas over about a onemonth period.
 7. A container according to claim 1, wherein said sealcomprises a weld integrally joining said closure to said wall structure.8. A container according to claim 7, wherein said weld is formed bymeans of an electron beam.
 9. A container for holding a mixture of aplurality of constituent gases, said gases having respectivepredetermined concentrations within said mixture, said container adaptedto maintain the concentrations of said gases within a predeterminedconcentration range over a predetermined time period, said containercomprising: a vessel having a wall structure defining an internal spacefor containing said gas, said wall structure comprising a metal havingthe property of being inert with respect to the constituent gases andhaving an opening therein for access to said internal space; a closurepositioned in overlying relation with said opening, a seal for attachingsaid closure to said wall structure, said closure and said seal havingthe same properties of being inert to the constituent gases as said wallstructure; and a valve mounted on said closure, said valve beingopenable and closable for permitting controlled release of said gasmixture from said vessel.
 10. A container according to claim 9, whereinsaid seal comprises a weld integrally joining said closure to said wallstructure.
 11. A container according to claim 10, wherein said weldmaintains a first one of said gases at a concentration within a range ofabout +1% to about −5% of said predetermined concentration of said firstgas over about a one month period.
 12. A container according to claim10, wherein said weld maintains a first one of said gases at aconcentration within a range of about +1% to about −7% of saidpredetermined concentration of said first gas over about a one monthperiod.
 13. A container according to claim 11, wherein said weldmaintains a second one of said gases at a concentration within a rangeof about +3% to about −15% of said predetermined concentration of saidsecond gas over about a one month period.
 14. A container according toclaim 11, wherein said weld maintains a second one of said gases at aconcentration within a range of about +3% to about −30% of saidpredetermined concentration of said second gas over about a one monthperiod.
 15. A container according to claim 11, wherein said weldmaintains a third one of said gases at a concentration within a range ofabout +3% to about −15% of said predetermined concentration of saidthird gas over about a one month period.
 16. A container according toclaim 11, wherein said weld maintains a third one of said gases at aconcentration within a range of about +3% to about −30% of saidpredetermined concentration of said third gas over about a one monthperiod.
 17. A container according to claim 10, wherein said weld isformed by means of an electron beam.
 18. A container according to claim10, wherein said vessel comprises a thin-walled aluminum cylinder, saidopening being positioned at one end thereof.
 19. A container accordingto claim 18, further comprising an outwardly turned lip surrounding saidopening, said closure comprising a flange extending around itsperimeter, said weld integrally joining said flange and said lip.
 20. Acontainer according to claim 19, wherein said flange comprises aperimeter having a curvature matched to the curvature of said lip.
 21. Acontainer according to claim 10, wherein said weld maintains a morereactive one of said gases at a concentration within a range of about+1% to about 5% of said predetermined concentration of said morereactive gas over about a one month period.
 22. A container according toclaim 10, wherein said weld maintains a more reactive one of said gasesat a concentration within a range of about +1% to about −7% of saidpredetermined concentration of said first gas over about a one monthperiod.
 23. A container according to claim 21, wherein said weldmaintains a less reactive one of said gases at a concentration within arange of about +3% to about −15% of said predetermined concentration ofsaid less reactive gas over about a one month period.
 24. A containeraccording to claim 21, wherein said weld maintains a less reactive oneof said gases at a concentration within a range of about +3% to about−30% of said predetermined concentration of said less reactive gas overabout a one month period.
 25. A container for holding a singleconstituent gas, said gas having a predetermined concentration ofimpurities upon filling, said container comprising: a vessel having awall structure defining an internal space for containing said gas, saidwall structure having an opening therein for access to said internalspace; a closure positioned in overlying relation with said opening, aseal for attaching said closure to said wall structure so as to maintainsaid impurities at a concentration within a range of about +1% to about−1% of said predetermined concentration of said impurities upon fillingover about a one month period; and a valve mounted on said closure, saidvalve being openable and closable for permitting controlled release ofsaid gas mixture from said vessel.
 26. A method of maintaining gasconcentration stability among constituent gases of a gas mixture withina predetermined concentration variation range and over a predeterminedtime period, said method comprising the steps of: providing a vessel forholding said gas mixture under pressure, said vessel having an openingtherein; providing a closure adapted to engage said vessel in overlyingrelation with said opening, said closure having a valve mounted thereonfor permitting controlled filling and release of said gas mixture to andfrom said vessel; and forming a seal between said closure and saidvessel, said seal, said closure and said vessel having the property ofbeing inert with respect to the constituent gases, said seal providing afluid tight joint between said closure and said vessel that maintains afirst one of said gases at a concentration of about +1% to about −5% ofa predetermined concentration of said first gas over about a one monthperiod.
 27. A method according to claim 26, further comprising the stepof forming a seal that maintains said first one of said gases at aconcentration within a concentration of about +1% to about −7% of saidpredetermined concentration of said first gas over about a one monthperiod.
 28. A method according to claim 26, further comprising the stepof forming a seal that maintains a second one of said gases at aconcentration within a range of about +3% to about −15% of saidpredetermined concentration of said second gas over about a one monthperiod.
 29. A method according to claim 26, further comprising the stepof forming a seal that maintains a second one of said gases at aconcentration of about +3% to about −30% of said predeterminedconcentration of said second gas over about a one month period.
 30. Amethod according to claim 26, further comprising the step of forming aseal that maintains a third one of said gases at a concentration withina range of about +3% to about −15% of said predetermined concentrationof said third gas over about a one month period.
 31. A method accordingto claim 26, further comprising the step of forming a seal thatmaintains a third one of said gases at a concentration of about +3% toabout −30% of said predetermined concentration of said third gas overabout a one month period.
 32. A method according to claim 26, whereinsaid seal is formed by the step of electron-beam welding said closure tosaid vessel.
 33. A method according to claim 26, further comprising thestep of filling said container with said gas mixture.
 34. A methodaccording to claim 33, wherein said filing step comprises filling saidcontainer with a gas mixture comprising bromopentafluorobenzene,trifluoromethyl benzene and nitrogen.